The present invention relates to a multi-carrier communication system, a multi-carrier transmitter for use in such a multi-carrier communication system, and a multi-carrier receiver for use in such a multi-carrier communication system.
Such a multi-carrier system, multi-carrier transmitter, and multi-carrier receiver are already known in the art, e.g. from the article ‘Very-High-Speed Digital Subscriber Lines’ from the authors John M. Cioffi, Vladimir Oksman, Jean-Jacques Werner, Thierry Pollet, Paul M. P. Spruyt, Jacky S. Chow and Krista S. Jacobsen. This article was published in IEEE Communications Magazine in April 1999, and describes from page 75, right-hand column to page 77, left-hand column, a communication system that combines multi-carrier modulation (DMT or Discrete Multi Tone modulation) with time division duplexing (TDD or ping-pong). In such a time division duplexing system, downstream transmission and upstream transmission occur in different time intervals. For downstream transmission from a line termination at the central office to a network termination at the customer premises, the line termination occupies a first time interval: the downstream frame. For upstream transmission from the network termination at the customer premises to the line termination at the central office, the network termination occupies a second time interval: the upstream frame. The time division duplexing (TDD) frame timing and the sample timing of the line termination and the network termination have to be synchronised. In a DMT (Discrete Multi Tone) based system like the one known from the above cited article, sample timing is synchronised between the line termination and network termination through transmission of a pilot carrier whose instantaneous frequency is a fraction of the sampling rate, which is orthogonal to other carriers transferred in the multi-carrier system, and which is used in the line termination or the network termination to control a rotor and skip/duplicate unit, such as is described in European Patent Application EP 0 820 171, entitled ‘Multicarrier transmitter or receiver with phase rotators’. To realise synchronisation of the TDD (Time Division Duplexing) frame timing, the clocks from which the TDD framing is derived at the line termination and at the network termination may be locked to an external clock, anywhere available, such as is proposed in U.S. Pat. No. 5,864,544 entitled ‘TDD Communication System Which Receives Timing Signals from a Satellite and Automatically, Successively, Repetitively, and Alternatingly Transmits and Receives Compressed Signals in a Single Channel’. In the TDD based communication system that is described therein, two communicating transceivers derive the TDD frame timing from a timing signal available through satellites such as the global positioning system (GPS) clock. If however in a multi-carrier, time division duplexing system, the TDD frame timing would be derived from an externally available clock like is done in U.S. Pat. No. 5,864,544, the jitter of the external clock would degrade the signal to noise ratio for transmission of all carriers used in the multi-carrier system. The effect of timing jitter on the performance of multi-carrier systems is analysed for example in the article ‘The Effect of Timing Jitter on the Performance of a Discrete Multitone System’ from the authors T. Nicholas Zogakis and John M. Cioffi, an article that was published in the magazine ‘IEEE Transactions on Communications’, Vol. 44, No. 7, July 1996.